This invention relates to a method and apparatus for controlling time diversity in radio telephony.
The invention has application, in particular, in personal satellite systems where power budgets are marginal, and also in terrestrial systems where signal levels can be at times insufficient, and in each system aims to improve the reliability of voice communication.
Satellite communications to hand-held terminals suffer from strong variations of the received signal power due to signal shadowing and multipath fading. Local shadowing of the satellite signal by obstacles in the propagation path, eg buildings, bridges, trees, results in attenuation over the total signal bandwidth. For low satellite elevation the shadowed areas are larger than for high elevation. Multipath fading occurs because the satellite signal is received not only via the direct path but also after being reflected from objects in the surroundings. Due to their different propagation distances, multipath signals can add destructively resulting in a deep fade. In addition to fade depth statistics, the distribution of fade duration has a major impact on the usability of the channel. Similar effects occur in both uplink and downlink directions although the statistics may vary, especially where the duplex distance in FDD transmissions is significantly large.
Therefore, for all types of land mobile satellite system, the communication link between the satellite and the mobile terminal is the most critical part of the total transmission path from earth-to-satellite-to-earth and limits the performance of the total system. Furthermore, hand-held systems are even more of a problem due to head effects and near-omni-directional antennas.
In most satellite radio environments applied to hand-held personal satellite services, high-frequency fading process is superimposed on the low-frequency shadowing process. Relatively xe2x80x9cgoodxe2x80x9d and very xe2x80x9cbadxe2x80x9d channel periods can be distinguished, having multiple dB mean level difference. The good channel state corresponds to areas with unobstructed view of the satellite (unshadowed areas) and is relatively free of multipath degradation. The bad channel state represents areas where the direct satellite signal is shadowed by obstacles and/or significantly affected by multipath fades.
In many existing systems two or more path diversity is used in order to improve system performance. The present invention is concerned with the variability of retransmission or redundancy of a single path.
Time diversity techniques also exist in known systems. One example of a known time diversity technique is applied in GSM, where the contents of a voice frame are spread over a number of TDMA slots by interleaving. The result is that the received signal may be reconstructed and the FEC information (forward error correction) that has been transmitted, together with the voice frame bits, is then used to correct any portions that may have been corrupted by multi-path fading or interference. The major disadvantage of such interleaving is that in order to successfully protect against serious levels of multi-path fading, deep interleaving must be used: ie, there is a substantial delay introduced in the voice communication. Long delays reduce the perceived quality due to the loss of interactivity in duplex conversations. Extreme examples of the effect of long delays are well known in multi-satellite paths used in some telecommunication circuits, although in this example the delay is set largely by the satellite path length rather than by deliberate interleaving and processing.
In another known time diversity technique, two (or more) representations of the speech signal are transmitted separated in time by an appropriate time dependent on the correlation characteristics of the radio path. In this example, the receiving equipment simply selects the least corrupted transmission on a slot-by-slot or frame-by-frame basis and reconstructs the audio path; the delay introduced in the audio path is constant and equal to at least the delay between the first and last samples.
More complex techniques may be added to either of the foregoing examples, or they may be combined in some way, and the transmission of secondary samples or associated FEC data may be suppressed if signalling indicates the first transmission is received successfully. The RACE ATDMA project employs such a technique based on ARQ Type II (Automatic Repeat Request). The advantage of selective transmission of secondary samples is that the demand on system/network resources (eg power and spectrum) is reduced.
It is also to be noted that the time-diverse components are normally transmitted to the receiver from the same transmitter, although the transmitter may employ xe2x80x9cantenna diversityxe2x80x9d, and Qualcomm CDMA employs a technique similar to this for micro-cell operation. The use of a second transmitter and/or antenna site provides path diversity rather than direct time diversity. However, if the second transmission is only made when the first transmission is found to be corrupted (ie ARQ), then the present invention can be applicable.
According to one aspect of the present invention, there is provided a method of communication in radio telephony using time diversity in the transmitted signal, according to which the time diversity is varied in accordance with radio propagation conditions from relatively long time delays under poor propagation conditions down to relatively short delays or zero time delay under good propagation conditions.
According to another aspect of the invention, there is provided, in a system of communication in radio telephony, apparatus comprising means at a transmitting station for the introduction of time diversity, means for detecting the quality of signal received at a receiver station, and means at the transmitting station for varying the time delay of the diversity in accordance with the detected quality of communication at the receiver station.
Thus, the essence of the invention is to provide a technique where the time diversity may be varied according to the actual radio propagation conditions, ranging from nil diversity to time delays normally considered too extreme for duplex voice operation in public networks: eg up to several seconds.
In simple terms, the time diversity is freely variable during speech and the overall speech delay similarly varied such that under more favourable conditions the speech delay, and user perception of such delay, is considered minimal or undetectable (normally considered to be less than 40 ms). Under worse conditions, the delay is increased to allow diversity gain to be applied to maintain at least intelligible communication. The concept of increasing delay is not too difficult to consider, as it is only necessary to insert a period of silence for example. Provided the required increase is progressive, and can be done over a period of several seconds, then the user""s perception and reaction need not be severe. The inserted delay thus needs to be carried out intelligently and, where possible, insertions made gradually by either inserting silence or preferably repeating selected frames at the end of words or at other less sensitive parts.
The concept of reducing delay when conditions improve is more radical, but it will be appreciated that it is possible to shorten a period of speech (of perhaps one or a few seconds) simply by omitting selected voice frames or selected voice patterns. Certain natural silence periods may also be omitted. Simple tests show that the loss of intelligibility by omitting selected portions of speech is minimal; the perceived loss of quality is largely unnoticed due to its transient nature, and this perception is preferable to complete loss of speech due to otherwise insufficient time delay diversity.